The invention relates to a method, an accelerator signal converter, an electronic control unit, a vehicle control system and a working machine for controlling a power source adapted to drive at least one ground engaging element of a working machine. The term “power source” is in the following exemplified by an internal combustion engine, such as a diesel engine. This should be regarded as a non-limiting example of such a power source.
Some drivers experience that working machines, such as wheel loaders, are hard to operate. The reason is that the engine in the working machine is used for powering both the hydraulic system and the traction system of the machine. Since the implement of the machine (such as a bucket) acts on a gravel pile or any other object that the machine works with, there will be a strong force coupling via both systems to the engine.
Consequently, the engine will counteract itself since the traction force counteract with the hydraulic forces. The traction force is transmitted to the pile from the engine to the wheels of the machine and further to the ground on which the pile rests. The hydraulic forces are transmitted from the engine to the implement and further to the pile This means that with higher traction force, the lifting force is reduced. In a similar way, the tilt operation by the hydraulic system is counteracted by the traction system.
FIG. 1 illustrates an example. The traction force (Force [kN]) is dependent on the engine speed. When the engine speed is increased, the traction force will as described reduce the effective lift force (LF) and tilt force (BF). The hydraulic force reduction is proportional with the traction force and quadratically proportional with the engine speed (in a driveline with hydrodynamic torque converter). The driver is therefore constantly challenged to balance the hydraulic system and the traction system by controlling the hydraulic levers and the accelerator pedal of the working machine.
Today there are solutions available which reduce the traction, so that the engine speed is limited to avoid that a certain traction force is exceeded. With such a solution the driver will not be able to increase the engine speed over a certain limit. That is, the driver's possibilities to manoeuvre the machine are restricted so that he is not able to decide fully how to operate the machine.
Another phenomena when for instance loading gravel is wheel skidding. When the torque/power, transmitted via the traction systems, exceeds the friction (connection between wheel and ground) the wheel/-s skid. This also influences the maneuvering possibilities for the driver in a negative way.
Furthermore, when the driver drives the machine at a constant speed, the machine may start oscillating. The reason for this is a coupling of accelerator pedal angle to engine speed demand and thus engine speed, and further via engine torque to machine traction force. The normal mapping of the accelerator pedal is a compromise. It may be that this mapping works very well when the driver drives the machine at low speed and low gear. The angle of the accelerator pedal results in a suitable machine speed. However, this mapping may work worse at higher engine speed and especially at a low gear. The high speed may cause the driver's chair to start swing which results in that the drivel's foot and therefore the pedal angle starts changing. The changed angle will inevitably results in a changed engine speed demand, which the engine will fulfil by increasing torque until the higher speed is reached.
In a driveline of the working machine without torque converter or with a torque converter with activated lock-up, the engine's oscillating torque is directly transferred to the wheels and creates an oscillating traction force. In both situations, this will finally result in the negative effect that the whole machine starts oscillating.
It is desirable to solve at least one of the above-mentioned problems by balancing the hydraulic system and the traction system, without restricting the possibility to operate the machine. It is also desirable to avoid oscillations when driving the machine at a high speed and to avoid wheel skidding when for instance loading gravel.
According to an aspect of the present invention a method is provided for controlling an power source adapted to drive at least one ground engaging element of a working machine. The method comprises the step of receiving an operator control input indicative of the control of the power source. What particularly characterizes the method is that it comprises the steps of:                receiving a state input indicative of an operating state of the machine,        determining an operation signal in the response to the operator control input and the operating state input,        sending the determined operation signal for controlling the power source accordingly.        
An aspect of the present invention also relates to an accelerator signal converter being adapted to perform any of the method steps according to the method. Furthermore, an aspect of the present invention relates to an Electronic Control Unit (ECU) comprising the accelerator signal converter, a vehicle control system comprising the ECU, and a working machine comprising the vehicle control system.
The main advantage with an aspect of the present invention is that the accelerator signal converter ultimately modifies the relationship between the accelerator pedal angle and the engine speed, which assists the driver when maneuvering the machine. This results in that the balancing of the engine torque between the hydraulic system and the traction system is easier to handle. The mapping can thereby be adapted for particular operating states, which gives the effect that that the engine control unit/entity is able to assist the driver in different working conditions.
Other preferred embodiments and advantages of the invention will emerge from the detailed description below.